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Abstract:

A display device includes a display portion having red, green, blue and
white sub pixels, a converter generating red, green and blue conversion
signals using a brightness ratio from a ratio storage, an upper limit
value calculator calculating an upper limit value of a display brightness
of the white sub pixel using the red, green and blue conversion signals
and the brightness ratio, a lower limit value calculator calculating a
lower limit value of the display brightness of the white sub pixel using
the red, green and blue conversion signals and the brightness ratio, and
a white control signal generator generating a white output control signal
for controlling the display brightness of the white sub pixel such that
the display brightness is not more than the upper limit value and not
less than the lower limit value, and outputting the generated white
output control signal to the display portion.

Claims:

1. A display device, comprising: a display portion that includes pixels
each having a red sub pixel, a green sub pixel, a blue sub pixel and a
white sub pixel for respectively displaying a red color, a green color, a
blue color and a white color, and displays a video picture corresponding
to an input video signal; a ratio storage that stores, as a brightness
ratio, a quotient obtained by dividing a brightness that can be displayed
with the white sub pixel by a brightness that can be displayed with
three-color sub pixels of the red sub pixel, the green sub pixel and the
blue sub pixel; a converter that converts the video signal using the
brightness ratio stored in the ratio storage to generate a red conversion
signal, a green conversion signal and a blue conversion signal; an upper
limit value calculator that calculates an upper limit value of a display
brightness of the white sub pixel using the red conversion signal, the
green conversion signal and the blue conversion signal generated by the
converter and the brightness ratio stored in the ratio storage; a lower
limit value calculator that calculates a lower limit value of the display
brightness of the white sub pixel using the red conversion signal, the
green conversion signal and the blue conversion signal generated by the
converter and the brightness ratio stored in the ratio storage; and a
white control signal generator that generates a white output control
signal for controlling the display brightness of the white sub pixel such
that the display brightness is not more than the upper limit value
calculated by the upper limit value calculator and not less than the
lower limit value calculated by the lower limit value calculator, and
outputs the generated white output control signal to the display portion.

2. The display device according to claim 1, further comprising: a factor
storage that stores, when at least one conversion signal among the red
conversion signal, the green conversion signal and the blue conversion
signal generated by the converter is used as a reference conversion
signal, an adjustment factor for adjusting a ratio of a signal level
representing the display brightness of the white sub pixel relative to a
signal level of the reference conversion signal; and a reference signal
calculator that calculates a reference signal representing the display
brightness of the white sub pixel using the reference conversion signal
and the adjustment factor, wherein the white control signal generator
generates the white output control signal for setting the display
brightness of the white sub pixel to the upper limit value calculated by
the upper limit value calculator when the display brightness represented
by the reference signal is more than the upper limit value, generates the
white output control signal for setting the display brightness of the
white sub pixel to the lower limit value calculated by the lower limit
value calculator when the display brightness represented by the reference
signal is less than the lower limit value, and uses the reference signal
as the white output control signal when the display brightness
represented by the reference signal is not more than the upper limit
value and not less than the lower limit value.

3. The display device according to claim 2, wherein, when the adjustment
factor stored in the factor storage is p, the adjustment factor is set to
be 0<p<1, and wherein the reference signal calculator uses a
result, which is obtained by multiplying the reference conversion signal
by the adjustment factor, as the reference signal.

4. The display device according to claim 1, further comprising: a color
control signal generator that generates a red output control signal, a
green output control signal and a blue output control signal for
respectively controlling display brightnesses of the red sub pixel, the
green sub pixel and the blue sub pixel using the white output control
signal generated by the white control signal generator, the red
conversion signal, the green conversion signal and the blue conversion
signal generated by the converter, and the brightness ratio stored in the
ratio storage, and outputs the generated red output control signal, green
output control signal and blue output control signal to the display
portion.

5. The display device according to claim 1, further comprising: a factor
storage that stores, when at least one conversion signal among the red
conversion signal, the green conversion signal and the blue conversion
signal generated by the converter is used as a reference conversion
signal, an adjustment factor for adjusting a ratio of a signal level
representing the display brightness of the white sub pixel relative to a
signal level of the reference conversion signal; and a white reference
signal calculator that calculates, when a pixel which is a control target
of a display brightness thereof is defined as a present pixel, a white
reference signal representing a display brightness of the white sub pixel
included in the present pixel using the reference conversion signal
corresponding to the present pixel, the reference conversion signal
corresponding to at least one peripheral pixel around the present pixel,
and the adjustment factor stored in the factor storage, wherein the white
control signal generator generates the white output control signal for
setting the display brightness of the white sub pixel to the upper limit
value calculated by the upper limit value calculator when the display
brightness represented by the white reference signal is more than the
upper limit value, generates the white output control signal for setting
the display brightness of the white sub pixel to the lower limit value
calculated by the lower limit value calculator when the display
brightness represented by the white reference signal is less than the
lower limit value, and uses the white reference signal as the white
output control signal when the display brightness represented by the
white reference signal is not more than the upper limit value and not
less than the lower limit value.

6. The display device according to claim 5, further comprising: a color
reference signal calculator that calculates a color reference signal
representing the display brightness of the white sub pixel for use in
controlling the red sub pixel, the green sub pixel and the blue sub pixel
included in the present pixel using the reference conversion signal
corresponding to the present pixel and the adjustment factor stored in
the factor storage; a white correction signal generator that generates a
white correction signal for use in controlling the red sub pixel, the
green sub pixel and the blue sub pixel included in the present pixel
using the color reference signal calculated by the color reference signal
calculator, the upper limit value calculated by the upper limit value
calculator, and the lower limit value calculated by the lower limit value
calculator; and a color control signal generator that generates a red
output control signal, a green output control signal and a blue output
control signal for respectively controlling display brightnesses of the
red sub pixel, the green sub pixel and the blue sub pixel included in the
present pixel using the white correction signal generated by the white
correction signal generator, the red conversion signal, the green
conversion signal and the blue conversion signal generated by the
converter, and the brightness ratio stored in the ratio storage, and
outputs the generated red output control signal, green output control
signal and blue output control signal to the display portion.

7. The display device according to claim 5, wherein the white reference
signal calculator calculates an interpolation conversion signal based on
interpolation calculation from the reference conversion signal
corresponding to the present pixel and the reference conversion signal
corresponding to the at least one peripheral pixel, and uses a product of
the interpolation conversion signal and the adjustment factor as the
white reference signal.

8. The display device according to claim 1, wherein the converter
generates, when the brightness ratio stored in the ratio storage is k,
the red conversion signal, the green conversion signal and the blue
conversion signal by respectively multiplying a red input signal, a green
input signal and a blue input signal as the input video signal by (1+k),
and wherein the upper limit value calculator sets, when a maximum value
of brightness that can be displayed by each of the sub pixels is defined
as a brightness maximum value, a conversion signal with a minimum signal
level among the red conversion signal, the green conversion signal and
the blue conversion signal as the upper limit value in a case where none
of the red conversion signal, the green conversion signal and the blue
conversion signal exceeds the brightness maximum value, and sets the
brightness maximum value as the upper limit value in a case where at
least one of the red conversion signal, the green conversion signal and
the blue conversion signal exceeds the brightness maximum value.

9. The display device according to claim 1, wherein the lower limit value
calculator sets, when a maximum value of brightness that can be displayed
by each of the sub pixels is defined as a brightness maximum value, zero
as the lower limit value in a case where none of the red conversion
signal, the green conversion signal and the blue conversion signal
exceeds the brightness maximum value, and sets a surplus relative to the
brightness maximum value of a conversion signal which exceeds the
brightness maximum value most as the lower limit value in a case where at
least one of the red conversion signal, the green conversion signal and
the blue conversion signal exceeds the brightness maximum value.

10. A display method for use in a display device having a display portion
that includes pixels each having a red sub pixel, a green sub pixel, a
blue sub pixel and a white sub pixel for respectively displaying a red
color, a green color, a blue color and a white color, and displays a
video picture corresponding to an input video signal, the method
comprising: a conversion step of converting the video signal using a
brightness ratio, which is a quotient obtained by dividing a brightness
that can be displayed with the white sub pixel by a brightness that can
be displayed with three-color sub pixels of the red sub pixel, the green
sub pixel and the blue sub pixel, to generate a red conversion signal, a
green conversion signal and a blue conversion signal; an upper limit
value calculation step of calculating an upper limit value of a display
brightness of the white sub pixel using the red conversion signal, the
green conversion signal and the blue conversion signal generated in the
conversion step and the brightness ratio; a lower limit value calculation
step of calculating a lower limit value of the display brightness of the
white sub pixel using the red conversion signal, the green conversion
signal and the blue conversion signal generated in the conversion step
and the brightness ratio; and a white control signal generation step of
generating a white output control signal for controlling the display
brightness of the white sub pixel such that the display brightness is not
more than the upper limit value calculated in the upper limit value
calculation step and not less than the lower limit value calculated in
the lower limit value calculation step, and outputting the generated
white output control signal to the display portion.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a display device having a display
portion such as a liquid crystal display or a plasma display panel, and
to a display method thereof.

BACKGROUND ART

[0002] As the foregoing display device, proposed is a display device which
displays a color video picture configured from the three primary colors
of red, green and blue with four colors on a display portion having
pixels including red sub pixels, green sub pixels, blue sub pixels and
white sub pixels (for example, refer to Patent Document 1). In the device
described in Patent Document 1, a minimum value among the
three-primary-color signals is generated as a white output control
signal, and this white output control signal and three-primary-color
output control signals obtained by subtracting the white output control
signal from the foregoing three-primary-color input video signals are
supplied to the display portion.

[0003] However, in the device described in Patent Document 1, since the
white sub pixels are always used for a color that can be reproduced using
the red sub pixels, the green sub pixels and the blue sub pixels or for a
color that can be reproduced using the white sub pixels, the red sub
pixels, the green sub pixels and the blue sub pixels become dark in
comparison to the white sub pixels, and there were cases where black
vertical streaks would become noticeable.

[0005] The present invention resolves the foregoing problems, and an
object of this invention is to provide a display device and a display
method capable of displaying a video picture in which black vertical
streaks are unnoticeable.

[0006] A display device according to the present invention includes: a
display portion that includes pixels each having a red sub pixel, a green
sub pixel, a blue sub pixel and a white sub pixel for respectively
displaying a red color, a green color, a blue color and a white color,
and displays a video picture corresponding to an input video signal; a
ratio storage that stores, as a brightness ratio, a quotient obtained by
dividing a brightness that can be displayed with the white sub pixel by a
brightness that can be displayed with three-color sub pixels of the red
sub pixel, the green sub pixel and the blue sub pixel; a converter that
converts the video signal using the brightness ratio stored in the ratio
storage to generate a red conversion signal, a green conversion signal
and a blue conversion signal; an upper limit value calculator that
calculates an upper limit value of a display brightness of the white sub
pixel using the red conversion signal, the green conversion signal and
the blue conversion signal generated by the converter and the brightness
ratio stored in the ratio storage; a lower limit value calculator that
calculates a lower limit value of the display brightness of the white sub
pixel using the red conversion signal, the green conversion signal and
the blue conversion signal generated by the converter and the brightness
ratio stored in the ratio storage; and a white control signal generator
that generates a white output control signal for controlling the display
brightness of the white sub pixel such that the display brightness is not
more than the upper limit value calculated by the upper limit value
calculator and not less than the lower limit value calculated by the
lower limit value calculator, and outputs the generated white output
control signal to the display portion.

[0007] A display method according to the present invention is a display
method for use in a display device having a display portion that includes
pixels each having a red sub pixel, a green sub pixel, a blue sub pixel
and a white sub pixel for respectively displaying a red color, a green
color, a blue color and a white color, and displays a video picture
corresponding to an input video signal, the method includes: a conversion
step of converting the video signal using a brightness ratio, which is a
quotient obtained by dividing a brightness that can be displayed with the
white sub pixel by a brightness that can be displayed with three-color
sub pixels of the red sub pixel, the green sub pixel and the blue sub
pixel, to generate a red conversion signal, a green conversion signal and
a blue conversion signal; an upper limit value calculation step of
calculating an upper limit value of a display brightness of the white sub
pixel using the red conversion signal, the green conversion signal and
the blue conversion signal generated in the conversion step and the
brightness ratio; a lower limit value calculation step of calculating a
lower limit value of the display brightness of the white sub pixel using
the red conversion signal, the green conversion signal and the blue
conversion signal generated in the conversion step and the brightness
ratio; and a white control signal generation step of generating a white
output control signal for controlling the display brightness of the white
sub pixel such that the display brightness is not more than the upper
limit value calculated in the upper limit value calculation step and not
less than the lower limit value calculated in the lower limit value
calculation step, and outputting the generated white output control
signal to the display portion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a block diagram showing the configuration of the display
device according to the first embodiment of the present invention.

[0009]FIG. 2 is a diagram schematically showing the respective signals
such as the input video signal.

[0010]FIG. 3 is a diagram schematically showing the respective signals
such as the input video signal.

[0011]FIG. 4 is a diagram schematically showing the respective signals
such as the input video signal.

[0012]FIG. 5 is a diagram describing the effect of setting the maximum
value of the display brightness of the white sub pixels.

[0013]FIG. 6 is a diagram describing the effect of setting the minimum
value of the display brightness of the white sub pixels.

[0014]FIG. 7 is a block diagram showing the configuration of the display
device according to the second embodiment of the present invention.

[0015]FIG. 8 is a block diagram showing the configuration of the display
device according to the third embodiment of the present invention.

[0016]FIG. 9 is a diagram describing the interpolation using the
conversion signals of the present pixels and the peripheral pixels.

[0017]FIG. 10 is a diagram describing the generation of an output control
signal in the third embodiment.

[0018] FIG. 11 is a diagram describing another example of the
interpolation using the conversion signals of the present pixels and the
peripheral pixels.

[0019]FIG. 12 is a diagram comparing the output control signals that are
generated in the first embodiment and the third embodiment.

[0020]FIG. 13 is a diagram comparing the output control signals that are
generated in the first embodiment and the third embodiment.

[0021]FIG. 14 is a diagram showing a comparative example in which the red
output control signal, the green output control signal, and the blue
output control signal are also generated in consideration of the
influence of the peripheral pixels.

DESCRIPTION OF EMBODIMENTS

[0022] Embodiments of the present invention are now described with
reference to the drawings.

First Embodiment

[0023]FIG. 1 is a block diagram showing the configuration of the display
device according to the first embodiment of the present invention. This
display device comprises, as shown in FIG. 1, a display portion 11, a
storage 12, a converter 13, an upper limit value calculator 14, a lower
limit value calculator 15, a first generator 16, a second generator 17
and a reference signal calculator 18.

[0024] The display portion 11 contains pixels arranged in a matrix. The
respective pixels include red sub pixels 21 for displaying a red color,
green sub pixels 22 for displaying a green color, blue sub pixels 23 for
displaying a blue color, and white sub pixels 24 for displaying a white
color. As the display portion 11, adopted may be various types of
displays such as a liquid crystal display, a plasma display panel, and an
organic EL (electro luminescence) display.

[0025] The storage 12 is configured, for example, from a hard disk or a
flash memory, and stores a brightness ratio k and an adjustment factor p
which are obtained in advance. The brightness ratio k is obtained as
follows. Specifically, foremost, a brightness Wcd is measured by
displaying all white sub pixels 24 of the display portion 11 at a maximum
light quantity. Subsequently, a brightness (Rcd+Gcd+Bcd) is measured by
displaying all red sub pixels 21, green sub pixels 22 and blue sub pixels
23 of the display portion 11 at a maximum light quantity, respectively.
The brightness ratio k is a quotient obtained by dividing the former by
the latter. In other words, the brightness ratio k is obtained based on
the following:

k=Wcd/(Rcd+Gcd+Bcd) (1).

The brightness ratio k obtained in advance as described above is stored
in the storage 12.

[0026] Moreover, the adjustment factor p represents the ratio of the red
sub pixels 21, the green sub pixels 22 and the blue sub pixels 23, and
the white sub pixels 24, used in displaying a video picture in the
display portion 11. More specifically, the adjustment factor p adjusts
the ratio of the signal level representing the display brightness of the
white sub pixels 24 relative to the signal level of a green conversion
signal G1 generated by the converter 13. The adjustment factor p is set
in advance and is stored in the storage 12. The adjustment factor p is
set to be 0<p<1 and, in this embodiment, is set to be p=0.5 for
instance.

[0027] The converter 13 converts the input video signals Ri, Gi, Bi based
on the brightness ratio k, to generate red, green and blue conversion
signals R1, G1, B1 for use in the display control of the red sub pixels
21, the green sub pixels 22 and the blue sub pixels 23. The upper limit
value calculator 14 calculates an upper limit value Wmx of the display
brightness of the white sub pixels 24 using the red, green and blue
conversion signals R1, G1, B1 and the brightness ratio k. The lower limit
value calculator 15 calculates a lower limit value Wmn of the display
brightness of the white sub pixels 24 using the upper limit value Wmx of
the display brightness of the white sub pixels 24, the red, green and
blue conversion signals R1, G1, B1 and the brightness ratio k.

[0028] The reference signal calculator 18 calculates a reference signal S
for use in the display control of the white sub pixels 24 using the green
conversion signal G1 and the adjustment factor p. The first generator 16
generates a white output control signal Wo for controlling the display
brightness of the white sub pixels 24 using the upper limit value Wmx,
the lower limit value Wmn and the reference signal S, and outputs the
generated white output control signal Wo to the second generator 17 and
the display portion 11. The second generator 17 generates red, green and
blue output control signals Ro, Go, Bo for controlling the display
brightness of the red sub pixels 21, the green sub pixels 22 and the blue
sub pixels 23 using the red, green and blue conversion signals R1, G1,
B1, the brightness ratio k and the white output control signal Wo, and
outputs the generated red, green and blue output control signals Ro, Go,
Bo to the display portion 11. In this embodiment, the green conversion
signal G1 corresponds to the reference conversion signal, the first
generator 16 corresponds to the white control signal generator, and the
second generator 17 corresponds to the color control signal generator.
Here, the converter 13, the upper limit value calculator 14, the lower
limit value calculator 15, the first generator 16, the second generator
17 and the reference signal calculator 18 are now described in further
detail with reference to FIGS. 2 to 4.

[0029] FIGS. 2 to 4 are diagrams schematically showing the respective
signals such as the input video signal. Sections (A) of FIGS. 2 to 4
respectively show examples of the input video signals Ri, Gi, Bi.
Sections (B) of FIGS. 2 to 4 respectively show examples of the conversion
signals R1, G1, B1. Sections (C) of FIGS. 2 to 4 respectively show
examples of the output control signals Ro, Go, Bo, Wo. Sections (D) of
FIGS. 2 to 4 respectively show the output control signals in the
comparative examples. Note that, in FIGS. 2 to 4, k=1 and p=0.5 are
illustrated for the sake of convenience of description.

[0031] In other words, the converter 13 generates the red conversion
signal R1, the green conversion signal G1, and the blue conversion signal
B1 by respectively increasing the input video signals Ri, Gi, Bi in an
amount that is equivalent to the brightness ratio k. The converter 13
generates the conversion signals R1, G1, B1 shown in section (B) of FIG.
2, for example, from the input video signals Ri, Gi, Bi shown in section
(A) of FIG. 2.

[0032] In FIGS. 2 to 4, "1" shows the maximum value of the displayable
brightness; that is, the maximum value of the range of gradation that can
be displayed by the red sub pixels 21, the green sub pixels 22 and the
blue sub pixels 23, and, when the signals are represented in 8 bits, the
maximum value corresponds to "255". In other words, in this embodiment,
the signals of the respective colors are shown by normalizing the maximum
value of the displayable brightness to "1". The conversion signals R1,
G1, B1 shown in sections (B) of FIGS. 2 to 4 represent the increase in
the brightness level that has become displayable based on the red sub
pixels 21, the green sub pixels 22 and the blue sub pixels 23 by
obtaining a brightness increasing effect based on the white sub pixels
24.

[0039] For example, in section (B) of FIG. 3, since R1/k=G1/k=B1/k>1,
the upper limit value Wmx will be:

Wmx=1

as shown in section (C) of FIG. 3 based on Formula (3). Meanwhile, since
R1/k=G1/k=B1/k>1, the lower limit value Wmn becomes a value that is
equivalent to the excess portion D1 relative to the brightness maximum
value of the R1/k, G1/k, B1/k as shown in section (C) of FIG. 3 based on
Formula (4).

[0040] For example, in section (B) of FIG. 4, since R1/k>1,
G1/k=B1/k=1, the upper limit value Wmx will be:

Wmx=1

as shown in section (C) of FIG. 4 based on Formula (3). Meanwhile, since
R1/k>1, the lower limit value Wmn becomes a value that is equivalent
to the excess portion D2 relative to the brightness maximum value of the
R1/k as shown in section (C) of FIG. 4 based on Formula (4).

[0041] As described above, as a result of using the excess portion of the
R1/k, G1/k, B1/k, which are obtained by respectively dividing the
conversion signals R1, G1, B1 by the brightness ratio k, relative to the
brightness maximum value as the lower limit value Wmn of the display
brightness of the white sub pixels 24, the portion that cannot be
represented as the display brightness due to the brightness maximum value
being exceeded can be reliably compensated with the display brightness of
the white sub pixels 24.

[0043] The first generator 16 uses the upper limit value Wmx as the white
output control signal Wo when the reference signal S exceeds the upper
limit value Wmx, uses the lower limit value Wmn as the white output
control signal Wo when the reference signal S is less than the lower
limit value Wmn, and uses the reference signal S as the white output
control signal Wo when the reference signal S is not more than the upper
limit value Wmx and not less than the lower limit value Wmn. In other
words, the first generator 16:

sets Wo=Wmx when S>Wmx,

sets Wo=Wmn when S<Wmn, and

sets Wo=S when Wmn≦S≦Wmx.

[0044] The second generator 17 generates the red, green and blue output
control signals Ro, Go, Bo based on Formula (6).

Ro=R1-k×Wo,

Go=G1-k×Wo,

Bo=B1-k×Wo (6)

[0045] In Formula (6), when k<1, in comparison to a case of directly
subtracting the white output control signal Wo, it is possible to inhibit
the red, green and blue output control signals Ro, Go, Bo from becoming
too dark.

[0046] Examples of the red, green, blue and white output control signals
Ro, Go, Bo, Wo generated as described above are respectively shown in
sections (C) of FIGS. 2 to 4. Meanwhile, sections (D) of FIGS. 2 to 4
show, as the comparative examples, the red, green and blue output control
signals obtained by generating a minimum value among the red, green and
blue conversion signals as the white output control signal, and
subtracting the white output control signal from the red, green and blue
conversion signals.

[0047] For example, when R1=G1=B1<1, k=1, and p=0.5 as shown in section
(B) of FIG. 2, Wo=S=p×G1=G 1/2, and Wo=Ro=Go=Bo as shown in section
(C) of FIG. 2. Meanwhile, in the comparative examples, as shown in
section (D) of FIG. 2, the minimum value among the red, green and blue
conversion signals is generated as the white output control signal.
Moreover, since the red, green and blue output control signals are
generated by subtracting the white output control signal from the red,
green and blue conversion signals, the red, green and blue output control
signals become zero. Even in the case shown in section (D) of FIG. 2, the
brightness is the same since k=1, and the colors are also reproduced.
Nevertheless, with the output control signal shown in section (D) of FIG.
2, there are cases where vertical streaks become noticeable in the video
picture displayed on the display portion 11.

[0048] For example, when R1=G1=B1>1, k=1, and p=0.5 as shown in section
(B) of FIG. 3, Wo=S=p×G1=G 1/2, and Wo=Ro=Go=Bo as shown in section
(C) of FIG. 3. Meanwhile, in the comparative examples, as shown in
section (D) of FIG. 3, the minimum value among the red, green and blue
conversion signals is generated as the white output control signal.
Moreover, the red, green and blue output control signals are generated by
subtracting the white output control signal from the red, green and blue
conversion signals. Accordingly, the red, green and blue output control
signals shown in section (D) are of a smaller value than the red, green
and blue output control signals shown in section (C). Accordingly, the
ratio of the red, green and blue output control signals Ro, Go, Bo and
the white output control signal Wo shown in section (C) of FIG. 3 is a
more appropriate value in comparison to the case shown in section (D).

[0049] In addition, for instance, when R1>1, G1=B1=1, k=1, and p=0.5 as
shown in section (B) of FIG. 4, S=p×G1<Wmn. Accordingly, Wo=Wmn,
Ro=R1-Wo=1, and Go=Bo>0 as shown in section (C) of FIG. 4. In other
words, the green and blue output control signals Go, Bo will not become
zero. Meanwhile, in the comparative examples, as shown in section (D) of
FIG. 4, the white output control signal is generated as 1, and the green
and blue output control signals become zero. Accordingly, the ratio of
the red, green and blue output control signals Ro, Go, Bo and the white
output control signal Wo shown in section (C) of FIG. 4 is also a more
appropriate value in comparison to the case shown in section (D).

[0050] As described above, in this embodiment, as shown in sections (C) of
FIGS. 2 to 4, the ratio of the red, green and blue output control signals
Ro, Go, Bo and the white output control signal Wo is an appropriate
value. Meanwhile, in the comparative examples shown in sections (D), the
ratios of the red, green and blue output control signals relative to the
white output control signal are small. Consequently, in the comparative
examples shown in sections (D), there are cases where vertical streaks
become noticeable in the video picture displayed on the display portion
11.

[0051]FIG. 5 is a diagram describing the effect of setting the upper
limit value Wmx of the display brightness of the white sub pixels 24.
FIG. 6 is a diagram describing the effect of setting the lower limit
value Wmn of the display brightness of the white sub pixels 24. The
effect of providing the upper limit value Wmx and the lower limit value
Wmn are now described with reference to FIGS. 5 and 6.

[0052] For example, the conversion signals R1, G1, B1 shown in section (B)
of FIG. 5 are obtained from the input video signals Ri, Gi, Bi shown in
section (A) of FIG. 5. In addition, let it be assumed that a white output
control signal greater than the upper limit value Wmx has been generated
from the conversion signals R1, G1, B1 as shown in section (C) of FIG. 5.
In this case, as shown in section (D) of FIG. 5, the color becomes light
since the ratios of the colors of red, green, blue become small. Thus, as
shown in section (E) of FIG. 5, preferably, the green and blue output
control signals are not made to be zero, and the ratios of the colors of
red, green, blue are not made to be too small.

[0053] Moreover, for example, the conversion signals R1, G1, B1 shown in
section (B) of FIG. 6 are obtained from the input video signals Ri, Gi,
Bi shown in section (A) of FIG. 6. In addition, let it be assumed that a
white output control signal that is smaller than the lower limit value
Wmn is generated as shown in section (C) of FIG. 6 from the conversion
signals R1, G1, B1. In this case, as shown in section (D) of FIG. 6, the
ratios of the colors of red, green, blue become small and the brightness
becomes insufficient. Thus, the color becomes light and becomes a dark
color. Therefore, as shown in section (E) of FIG. 6, preferably, the
white output control signal is increased to an appropriate level to
ensure the display brightness.

[0054] As described above, according to the first embodiment, the
reference signal S is generated using the adjustment factor p
representing the ratio of the red, green and blue output control signals
Ro, Go, Bo and the white output control signal Wo for use in display, and
the white output control signal Wo is generated using this reference
signal S. Accordingly, the ratio of the red, green and blue output
control signals Ro, Go, Bo and the white output control signal Wo for use
in display can be set appropriately. As a result, it is possible to
prevent the brightness of the red, green and blue sub pixels 21 to 23
from becoming too small relative to the brightness of the white sub
pixels 24, and to prevent vertical streaks from becoming noticeable in
the video picture displayed on the display portion 11.

[0055] Moreover, according to the first embodiment, the upper limit value
Wmx and the lower limit value Wmn of the display brightness of the white
sub pixels 24 are calculated by the upper limit value calculator 14 and
the lower limit value calculator 15, and the white output control signal
Wo is generated such that it is not more than the upper limit value Wmx
and not less than the lower limit value Wmn of the display brightness of
the white sub pixels 24. Accordingly, it is possible to prevent the color
from becoming light or becoming dark. Consequently, it is possible to
display, on the display portion 11, a high quality video picture which
faithfully reproduces the colors of the input video signals.

Second Embodiment

[0056]FIG. 7 is a block diagram showing the configuration of the display
device according to the second embodiment of the present invention. In
the second embodiment, the same symbol is assigned to the same element as
the first embodiment. The display device according to the second
embodiment shown in FIG. 7 does not comprise the reference signal
calculator 18 and comprises a first generator 161 as substitute for the
first generator 16, in the display device according to the first
embodiment shown in FIG. 1. The second embodiment is now described mainly
regarding the differences with the first embodiment.

[0057] The first generator 161 retains a factor A that is set in advance,
generates the white output control signal Wo based on Formula (7) using
the factor A, and outputs the generated white output control signal Wo to
the second generator 17 and the output portion 11.

Wo=A×Wmx+(1-A)×Wmn (7)

[0058] The factor A is set to be 0<A<1 and, for instance, is set to
be A=0.5. In this embodiment, the first generator 161 corresponds to the
white control signal generator, and the second generator 17 corresponds
to the color control signal generator.

[0059] In the second embodiment also, the upper limit value Wmx and the
lower limit value Wmn of the display brightness of the white sub pixels
24 are calculated by the upper limit value calculator 14 and the lower
limit value calculator 15, and the white output control signal Wo is
generated such that it is not more than the upper limit value Wmx and not
less than the lower limit value Wmn of the display brightness of the
white sub pixels 24. Accordingly, as with the foregoing first embodiment,
it is possible to prevent the color from becoming light or becoming dark.
Consequently, it is possible to display, on the display portion 11, a
high quality video picture which faithfully reproduces the colors of the
input video signals.

Third Embodiment

[0060]FIG. 8 is a block diagram showing the configuration of the display
device according to the third embodiment of the present invention. In the
third embodiment, the same symbol is assigned to the same element as the
first embodiment. The display device according to the third embodiment
shown in FIG. 8 comprises a first generator 162 as substitute for the
first generator 16, comprises a second generator 171 as substitute for
the second generator 17, and additionally comprises a white reference
signal calculator 181 and a third generator 19, in the display device
according to the first embodiment shown in FIG. 1. The third embodiment
is now described mainly regarding the differences with the first
embodiment.

[0061] The white reference signal calculator 181 calculates, when a
control target of the output control signal is defined as a present
pixel, a white reference signal Sw(i) for use in the display control of
the white sub pixels 24 included in the present pixel using the green
conversion signal G1 corresponding to the present pixel, the green
conversion signal G1 corresponding to at least one (for instance, three
in this embodiment) peripheral pixel positioned around the present pixel,
and the adjustment factor p. The white reference signal calculator 181 is
now described in further detail with reference to FIG. 9.

[0062]FIG. 9 is a diagram describing the interpolation using the
conversion signals of the present pixel and the peripheral pixels. The
white reference signal calculator 181 performs bicubic interpolation
based on Formula (8) to generate the interpolation value G1ave(i).

[0064] Here, G1(i) is the green conversion signal of the present pixel
SP(i), G1(i-1) is the green conversion signal of the pixel SP(i-1)
adjacent to the present pixel SP(i), G1(i+1) is the green conversion
signal of the pixel SP(i+1) adjacent to the present pixel SP(i) on a side
that is opposite to the pixel SP(i-1), and G1(i+2) is the green
conversion signal of the pixel SP(i+2) adjacent to the pixel SP(i+1).
Moreover, a, b, c, d are predetermined factors and are set, for example,
as a+b+c+d=1. Note that the factors a, b, c, d are set, for example, as
a=d, b=c when the widths of the red sub pixels 21, the green sub pixels
22, the blue sub pixels 23, and the white sub pixels 24 are equal in the
display portion 11.

[0065] Returning to FIG. 8, the first generator 162 generates the white
output control signal Wo for controlling the display brightness of the
white sub pixels 24 using the upper limit value Wmx, the lower limit
value Wmn and the white reference signal Sw(i), and outputs the generated
white output control signal Wo to the display portion 11. The first
generator 162 generates the white output control signal Wo in the same
manner as the first generator 16 of the first embodiment other than using
the white reference signal Sw(i). In other words, the first generator
162:

sets Wo=Wmx when Sw(i)>Wmx,

sets Wo=Wmn when Sw(i)<Wmn, and

sets Wo=Sw(i) when Wmn≦Sw(i)≦Wmx.

[0066] The third generator 19 generates the white correction signal Wc for
controlling the red sub pixels 21, the green sub pixels 22 and the blue
sub pixels 23 using the upper limit value Wmx, the lower limit value Wmn
and the reference signal S, and outputs the generated white correction
signal Wc to the second generator 171. The third generator 19 generates
the white correction signal Wc in the same manner as the first generator
16 of the first embodiment generates the white output control signal Wo.
In other words, the third generator 19:

sets Wc=Wmx when S>Wmx,

sets Wc=Wmn when S<Wmn, and

sets Wc=S when Wmn≦S≦Wmx.

[0067] The second generator 171 generates the red, green and blue output
control signals Ro, Go, Bo for controlling the display brightness of the
red sub pixels 21, the green sub pixels 22 and the blue sub pixels 23
using the red, green and blue conversion signals R1, G1, B1, the
brightness ratio k, and the white correction signal Wc, and outputs the
generated red, green and blue output control signals Ro, Go, Bo to the
display portion 11. The second generator 171 generates the red, green and
blue output control signals Ro, Go, Bo in the same manner as the second
generator 17 of the first embodiment other than using the white
correction signal Wc. In other words, the second generator 171 generates
the red, green and blue output control signals Ro, Go, Bo based on
Formula (10).

Ro=R1-k×Wc,

Go=G1-k×Wc,

Bo=B1-k×Wc (10)

[0068] In this embodiment, the green conversion signal G1 corresponds to
the reference conversion signal, the reference signal calculator 18
corresponds to the color reference signal calculator, the first generator
162 corresponds to the white control signal generator, the second
generator 171 corresponds to the color control signal generator, and the
third generator 19 corresponds to the white correction signal generator.

[0069]FIG. 10 is a diagram describing the generation of an output control
signal in the third embodiment. As shown in FIG. 10, the video signals of
the adjacent pixels SP(i-1), SP(i), SP(i+1), SP(i+2) are used among the
input video signals. Let it be assumed that the present pixel which is a
calculation target is the pixel SP(i). The generation of the output
control signal in the third embodiment is now described with reference to
FIGS. 8 and 10.

[0070] The white reference signal calculator 181 calculates the
interpolation value G1ave(i) using the green conversion signals G1(i-1),
G1(i), G1(i+1), G1(i+2) generated by the converter 13 from the input
video signal, and multiplies the interpolation value G1ave(i) by the
adjustment factor p to calculate the white reference signal Sw(i). The
first generator 162 generates the white output control signal Wo(i) of
the present pixel using the white reference signal Sw(i). Meanwhile, the
second generator 171 generates the red, green and blue output control
signals Ro(i), Go(i), Bo(i) of the present pixel using the reference
signal S and the white correction signal We that were calculated by using
only the green conversion signal G1(i) of the present pixel SP(i) as with
the first embodiment.

[0071] In the third embodiment, the white reference signal calculator 181
performs bicubic interpolation to generate the interpolation value
G1ave(i), but the present invention is not limited thereto. For example,
as shown in FIG. 11, one peripheral pixel may be used to generate the
interpolation value G1ave(i).

[0072] FIG. 11 is a diagram describing another example of the
interpolation using the conversion signals of the present pixel and the
peripheral pixel. In FIG. 11, the white reference signal calculator 181
generates the interpolation value G1ave(i) based on Formula (11).

G1ave(i)={G1(i)+G1(i+1)}/2 (11)

[0073] In other words, in FIG. 11, the white reference signal calculator
181 calculates, as the interpolation value G1ave(i), the average value of
the conversion signal G1(i) of the present pixel SP(i) and the green
conversion signal G1(i+1) of the peripheral pixel SP(i+1) adjacent to the
present pixel SP(i). With the interpolation shown in FIG. 11 also, the
white reference signal calculator 181 can favorably calculate the white
reference signal Sw(i) of the present pixel.

[0074] FIGS. 12 and 13 are diagrams comparing the output control signals
that are generated in the first embodiment and the third embodiment.
Section (A) of FIG. 12 shows an example of the input video signals.
Section (B) of FIG. 12 shows the output controls signals generated in the
first embodiment relative to the input video signals of section (A) of
FIG. 12. Section (C) of FIG. 12 shows the output control signals
generated in the third embodiment relative to the input video signals of
section (A) of FIG. 12. Section (A) of FIG. 13 shows another example of
the input video signals. Section (B) of FIG. 13 shows the output control
signals generated in the first embodiment relative to the input video
signals of section (A) of FIG. 13. Section (C) of FIG. 13 shows the
output control signals generated in the third embodiment relative to the
input video signals of section (A) of FIG. 13.

[0075] In the example shown in FIG. 12, as shown in section (A), the
colors of red, green, blue are of gray input video signals having the
same brightness. However, the magnitude relationship of the brightnesses
P(i-1), P(i), P(i+1), P(i+2) of the respective pixels is
P(i-1)=P(i)<P(i+1)=P(i+2). Relative to these input video signals, in
the first embodiment, as shown in section (B), a white output control
signal having the same brightness as the red, green and blue output
control signals is generated. This is the same as the example shown in
sections (C) of FIGS. 2 and 3.

[0076] Meanwhile, in the third embodiment, as a result of performing
bicubic interpolation using the brightness of the peripheral pixels, the
white output control signal Wo is generated in consideration of the
influence of the peripheral pixels. Consequently, as shown in section
(C), the brightness Wo(i) of the white output control signal of the pixel
P(i) is:

Bo(i)<Wo(i)<Ro(i+1)

relative to the brightness Bo(i) of the blue output control signal of the
pixel P(i) and the brightness Ro(i+1) of the red output control signal of
the pixel P(i+1). Accordingly, in comparison to the first embodiment
shown in section (B), it is possible to display an even smoother video
picture on the display portion 11.

[0077] In the example shown in FIG. 13, as with FIG. 12, the colors of
red, green, blue are of gray input video signals having the same
brightness. However, as shown in section (A) of FIG. 13, the magnitude
relationship of the brightnesses P(i-1), P(i), P(i+1), P(i+2) of the
respective pixels is P(i-1)<P(i)=P(i+1)>P(i+2). Relative to these
input video signals, in the first embodiment, as shown in section (B), a
white output control signal having the same brightness as the red, green
and blue output control signals is generated. This is the same as the
example shown in sections (C) of FIGS. 2 and 3.

[0078] Meanwhile, in the third embodiment, as a result of performing
bicubic interpolation using the brightness of the peripheral pixels, the
white output control signal Wo is generated in consideration of the
influence of the peripheral pixels. Consequently, as shown in section
(C), the brightness Wo(i-1) of the white output control signal of the
pixel P(i-1) is:

Bo(i-1)<Wo(i-1)<Ro(i)

relative to the brightness Bo(i-1) of the blue output control signal of
the pixel P(i-1) and the brightness Ro(i) of the red output control
signal of the pixel P(i). Moreover, the brightness Wo(i) of the white
output control signal of the pixel P(i) is:

Bo(i)<Wo(i)>Ro(i+1)

relative to the brightness Bo(i) of the blue output control signal of the
pixel P(i) and the brightness Ro(i+1) of the red output control signal of
the pixel P(i+1). Moreover, the brightness Wo(i+1) of the white output
control signal of the pixel P(i+1) is:

Bo(i+1)<Wo(i+1)<Ro(i+2)

relative to the brightness Bo(i+1) of the blue output control signal of
the pixel P(i+1) and the brightness Ro(i+2) of the red output control
signal of the pixel P(i+2). Accordingly, in comparison to the first
embodiment shown in section (B), it is possible to display an even
smoother video picture on the display portion 11.

[0079]FIG. 14 is a diagram showing, unlike the third embodiment, a
comparative example in which the red output control signal, the green
output control signal, and the blue output control signal are also
generated in consideration of the influence of the peripheral pixels.
Section (A) of FIG. 14 shows an example of the input video signals.
Section (B) of FIG. 14 shows the output control signals of the
comparative example that are generated relative to the input video
signals of section (A) of FIG. 14. As described above, in the third
embodiment, only the white output control signal is generated in
consideration of the influence of the peripheral pixels, and the red,
green and blue output control signals are generated only from the signals
of the present pixel without consideration of the peripheral pixels as in
the first embodiment. Meanwhile, the comparative example in which the
red, green and blue output control signals are also generated in
consideration of the influence of the peripheral pixels is now described
with reference to FIG. 14.

[0080] As shown in section (A) of FIG. 14, the colors of red, green, blue
are of gray input video signals having the same brightness. However, the
magnitude relationship of the brightnesses P(i-1), P(i), P(i+1), P(i+2)
of the respective pixels is P(i-1)=P(i)<P(i+1)=P(i+2). Since the white
output control signal is generated in consideration of the influence of
the peripheral pixels relative to these input video signals, as shown in
section (B) of FIG. 14, the brightness W(i) of the white output control
signal of the pixel P(i) is an intermediate value of the brightnesses
P(i-1), P(i+1) of the adjacent pixels as with the case shown in section
(C) of FIG. 12.

[0081] Meanwhile, in FIG. 14, the red, green and blue output control
signals are also generated in consideration of the influence of the
peripheral pixels. Accordingly, as shown in section (B) of FIG. 14, the
brightnesses R(i), G(i), B(i) of the red, green and blue output control
signals of the pixel P(i) are lower than the brightnesses P(i-1), P(i+1)
of the adjacent pixels. Thus, a discontinuous unevenness will occur in
the video picture displayed on the display portion 11. Accordingly, the
third embodiment which generates the red, green and blue output control
signals only based on the signals of the present pixel without
consideration of the influence of the peripheral pixels is more
preferable.

[0082] As described above, according to the third embodiment, the white
output control signal Wo is generated by using the signals of the
peripheral pixels positioned around the present pixel in addition to the
present pixel which is the calculation target. Accordingly, it is
possible to display an even smoother video picture on the display portion
11. Moreover, according to the third embodiment, the red, green and blue
output control signals Ro, Go, Bo are generated by using only the signals
of the present pixel. Accordingly, there is an advance in that the video
picture displayed on the display portion 11 will be free of discontinuous
unevenness.

[0083] (Other)

[0084] Note that, the reference signal S is calculated using the green
conversion signal G1 in the foregoing first embodiment, and the white
reference signal Sw(i) is calculated using the green conversion signal
G1(i) and the like in the foregoing third embodiment, but the present
invention is not limited thereto. Alternatively, as substitute for the
green conversion signal G1, the red conversion signal R1 or the blue
conversion signal B1 may also be used for instance. However, since green
has a higher contribution ratio relative to the brightness in comparison
to red and blue, the green conversion signal G1 is preferably used.
Moreover, alternatively, the brightness
Y(=mR×R1+mG×G1+mB×B1) obtained from the red conversion
signal R1, the green conversion signal G1 and the blue conversion signal
B1 may also be used. Here, mR, mG, mB are well-known conversion factors.
The brightness Y is suitable as the contribution ratio relative to the
display brightness.

[0085] Moreover, in each of the foregoing embodiments, while the
three-primary-color signals Ri, Gi, Bi are used as the input video
signals, there is no limitation to the signals that are input from the
outside for example. In other words, when a video signal such as NTSC
other than a three-primary-color signal is input from the outside, the
three-primary-color signals Ri, Gi, Bi may be separated from the video
signal which is input and used as the input video signals.

[0086] Moreover, a video signal often has gamma characteristics during the
filming. In this case, by converting the Ri, Gi, Bi signals into inverse
gamma, a value that is linearly proportional to the light quantity may be
used as the input video signal. In addition, the signal may be converted
into a signal having gamma characteristics once again upon outputting
that signal to the display portion 11. As a result of using a value that
is proportional to the light quantity in the display device of this
embodiment, it is possible to reproduce the colors even more faithfully.

[0087] Moreover, in each of the foregoing embodiments, while the
respective sub pixels 21 to 24 are displayed at a maximum light quantity
to obtain the brightness ratio k, the present invention is not limited
thereto. For example, the respective sub pixels 21 to 24 may be displayed
at a constant light quantity that is set in advance to obtain the
brightness ratio k.

[0088] Moreover, in each of the foregoing embodiments, the respective
formulae used in the converter 13, the upper limit value calculator 14,
the lower limit value calculator 15, the first generators 16, 161, 162,
the second generators 17, 171 and so on are merely examples, and
additional corrections may be added based on each of the foregoing
formulae. In other words, in each of the foregoing embodiments, in order
to simplify the description, while the colors to be represented by the
white sub pixels 24 are the same as the colors when respectively causing
the red sub pixels 21, the green sub pixels 22 and the blue sub pixels 23
to emit light at the same brightness, but in reality it is rare that such
conditions are ever satisfied. In fact, the brightness and the
chromaticity value of the white sub pixels 24 differ from those when
respectively causing the red sub pixels 21, the green sub pixels 22 and
the blue sub pixels 23 to emit light at the same brightness. However, in
the foregoing case, corrective calculation may be performed based on
known methods such as the balance factor or matrix operation in
accordance with the actual chromaticity value of the white sub pixels 24,
and the present invention is not limited to the respective formulae
described in each of the foregoing embodiments.

[0089] Note that the specific embodiments described above mainly include
the invention configured as described below.

[0090] A display device according to the present invention has: a display
portion that includes pixels each having a red sub pixel, a green sub
pixel, a blue sub pixel and a white sub pixel for respectively displaying
a red color, a green color, a blue color and a white color, and displays
a video picture corresponding to an input video signal; a ratio storage
that stores, as a brightness ratio, a quotient obtained by dividing a
brightness that can be displayed with the white sub pixel by a brightness
that can be displayed with three-color sub pixels of the red sub pixel,
the green sub pixel and the blue sub pixel; a converter that converts the
video signal using the brightness ratio stored in the ratio storage to
generate a red conversion signal, a green conversion signal and a blue
conversion signal; an upper limit value calculator that calculates an
upper limit value of a display brightness of the white sub pixel using
the red conversion signal, the green conversion signal and the blue
conversion signal generated by the converter and the brightness ratio
stored in the ratio storage; a lower limit value calculator that
calculates a lower limit value of the display brightness of the white sub
pixel using the red conversion signal, the green conversion signal and
the blue conversion signal generated by the converter and the brightness
ratio stored in the ratio storage; and a white control signal generator
that generates a white output control signal for controlling the display
brightness of the white sub pixel such that the display brightness is not
more than the upper limit value calculated by the upper limit value
calculator and not less than the lower limit value calculated by the
lower limit value calculator, and outputs the generated white output
control signal to the display portion.

[0091] According to the foregoing configuration, the display portion
includes pixels each having a red sub pixel, a green sub pixel, a blue
sub pixel and a white sub pixel for respectively displaying a red color,
a green color, a blue color and a white color, and displays a video
picture corresponding to an input video signal. The ratio storage stores,
as a brightness ratio, a quotient obtained by dividing a brightness that
can be displayed with the white sub pixel by a brightness that can be
displayed with three-color sub pixels of the red sub pixel, the green sub
pixel and the blue sub pixel. The converter converts the video signal
using the brightness ratio stored in the ratio storage to generate a red
conversion signal, a green conversion signal and a blue conversion
signal. The upper limit value calculator calculates an upper limit value
of a display brightness of the white sub pixel using the red conversion
signal, the green conversion signal and the blue conversion signal
generated by the converter and the brightness ratio stored in the ratio
storage. The lower limit value calculator calculates a lower limit value
of the display brightness of the white sub pixel using the red conversion
signal, the green conversion signal and the blue conversion signal
generated by the converter and the brightness ratio stored in the ratio
storage. The white control signal generator generates a white output
control signal for controlling the display brightness of the white sub
pixel such that the display brightness is not more than the upper limit
value and not less than the lower limit value, and outputs the generated
white output control signal to the display portion. Accordingly, the
display brightness of the white sub pixel can be controlled to be an
appropriate level that is not more than the upper limit value and not
less than the lower limit value. As a result, it is possible to display a
video picture in which black vertical streaks are unnoticeable.

[0092] Moreover, in the foregoing display device, preferably, the display
device further includes: a factor storage that stores, when at least one
conversion signal among the red conversion signal, the green conversion
signal and the blue conversion signal generated by the converter is used
as a reference conversion signal, an adjustment factor for adjusting a
ratio of a signal level representing the display brightness of the white
sub pixel relative to a signal level of the reference conversion signal;
and a reference signal calculator that calculates a reference signal
representing the display brightness of the white sub pixel using the
reference conversion signal and the adjustment factor, wherein the white
control signal generator generates the white output control signal for
setting the display brightness of the white sub pixel to the upper limit
value calculated by the upper limit value calculator when the display
brightness represented by the reference signal is more than the upper
limit value, generates the white output control signal for setting the
display brightness of the white sub pixel to the lower limit value
calculated by the lower limit value calculator when the display
brightness represented by the reference signal is less than the lower
limit value, and uses the reference signal as the white output control
signal when the display brightness represented by the reference signal is
not more than the upper limit value and not less than the lower limit
value.

[0093] According to the foregoing configuration, the factor storage
stores, when at least one conversion signal among the red conversion
signal, the green conversion signal and the blue conversion signal
generated by the converter is used as a reference conversion signal, an
adjustment factor for adjusting a ratio of a signal level representing
the display brightness of the white sub pixel relative to a signal level
of the reference conversion signal. The reference signal calculator
calculates a reference signal representing the display brightness of the
white sub pixel using the reference conversion signal and the adjustment
factor. The white control signal generator generates the white output
control signal for setting the display brightness of the white sub pixel
to the upper limit value when the display brightness represented by the
reference signal is more than the upper limit value calculated by the
upper limit value calculator. The white control signal generator
generates the white output control signal for setting the display
brightness of the white sub pixel to the lower limit value when the
display brightness represented by the reference signal is less than the
lower limit value calculated by the lower limit value calculator. The
white control signal generator uses the reference signal as the white
output control signal when the display brightness represented by the
reference signal is not more than the upper limit value and not less than
the lower limit value. Accordingly, since the reference signal that is
calculated using the adjustment factor for adjusting the ratio of the
signal level representing the display brightness of the white sub pixel
relative to the signal level of the reference conversion signal is used
as the white output control signal when the display brightness
represented by the reference signal is not more than the upper limit
value and not less than the lower limit value, it is possible to display
a video picture in which black vertical streaks are even more
unnoticeable.

[0094] In the foregoing display device, preferably, when the adjustment
factor stored in the factor storage is p, the adjustment factor is set to
be 0<p<1, and the reference signal calculator uses a result, which
is obtained by multiplying the reference conversion signal by the
adjustment factor, as the reference signal.

[0095] According to the foregoing configuration, the adjustment factor p
stored in the factor storage is set to be 0<p<1. The reference
signal calculator uses a result, obtained by multiplying the reference
conversion signal by the adjustment factor, as the reference signal.
Accordingly, since the reference signal is calculated as a value that is
smaller than the reference conversion signal, as a result of setting the
adjustment factor to be an appropriate value, it is possible to display a
video picture in which black vertical streaks are even more unnoticeable.

[0096] Moreover, in the foregoing display device, preferably, the display
device further includes a color control signal generator that generates a
red output control signal, a green output control signal and a blue
output control signal for respectively controlling display brightnesses
of the red sub pixel, the green sub pixel and the blue sub pixel using
the white output control signal generated by the white control signal
generator, the red conversion signal, the green conversion signal and the
blue conversion signal generated by the converter, and the brightness
ratio stored in the ratio storage, and outputs the generated red output
control signal, green output control signal and blue output control
signal to the display portion.

[0097] According to the foregoing configuration, the color control signal
controller generates a red output control signal, a green output control
signal and a blue output control signal for respectively controlling
display brightnesses of the red sub pixel, the green sub pixel and the
blue sub pixel using the white output control signal generated by the
white control signal generator, the red conversion signal, the green
conversion signal and the blue conversion signal generated by the
converter, and the brightness ratio stored in the ratio storage, and
outputs the generated red output control signal, green output control
signal and blue output control signal to the display portion.
Accordingly, since the red output control signal, the green output
control signal and the blue output control signal are generated using the
display brightness of the white sub pixel set to an appropriate level
which is not more than the upper limit value and not less than the lower
limit value, the display brightness of the red sub pixel, the green sub
pixel and the blue sub pixel can also be controlled to be an appropriate
level. As a result, it is possible to display a smooth video picture in
which black vertical streaks are unnoticeable.

[0098] Moreover, in the foregoing display device, preferably, the display
device further includes: a factor storage that stores, when at least one
conversion signal among the red conversion signal, the green conversion
signal and the blue conversion signal generated by the converter is used
as a reference conversion signal, an adjustment factor for adjusting a
ratio of a signal level representing the display brightness of the white
sub pixel relative to a signal level of the reference conversion signal;
and a white reference signal calculator that calculates, when a pixel
which is a control target of a display brightness thereof is defined as a
present pixel, a white reference signal representing a display brightness
of the white sub pixel included in the present pixel using the reference
conversion signal corresponding to the present pixel, the reference
conversion signal corresponding to at least one peripheral pixel around
the present pixel, and the adjustment factor stored in the factor
storage, wherein the white control signal generator generates the white
output control signal for setting the display brightness of the white sub
pixel to the upper limit value calculated by the upper limit value
calculator when the display brightness represented by the white reference
signal is more than the upper limit value, generates the white output
control signal for setting the display brightness of the white sub pixel
to the lower limit value calculated by the lower limit value calculator
when the display brightness represented by the white reference signal is
less than the lower limit value, and uses the white reference signal as
the white output control signal when the display brightness represented
by the white reference signal is not more than the upper limit value and
not less than the lower limit value.

[0099] According to the foregoing configuration, the factor storage
stores, when at least one conversion signal among the red conversion
signal, the green conversion signal and the blue conversion signal
generated by the converter is used as a reference conversion signal, an
adjustment factor for adjusting a ratio of a signal level representing
the display brightness of the white sub pixel relative to a signal level
of a reference conversion signal. The white reference signal calculator
calculates, when a pixel which is a control target of a display
brightness thereof is defined as a present pixel, a white reference
signal representing a display brightness of the white sub pixel included
in the present pixel using the reference conversion signal corresponding
to the present pixel, the reference conversion signal corresponding to at
least one peripheral pixel around the present pixel, and the adjustment
factor stored in the factor storage. The white control signal generator
generates the white output control signal for setting the display
brightness of the white sub pixel to the upper limit value when the
display brightness represented by the reference signal is more than the
upper limit value calculated by the upper limit value calculator. The
white control signal generator generates the white output control signal
for setting the display brightness of the white sub pixel to the lower
limit value when the display brightness represented by the reference
signal is less than the lower limit value calculated by the lower limit
value calculator. The white control signal generator uses the reference
signal as the white output control signal when the display brightness
represented by the reference signal is not more than the upper limit
value and not less than the lower limit value. Accordingly, a white
output control signal that gives consideration to the peripheral pixel
can be obtained as the white output control signal of the white sub pixel
included in the present pixel. As a result, it is possible to display, on
the display portion, an even smoother video picture with higher
definition.

[0100] Moreover, in the foregoing display device, preferably, the display
device further includes: a color reference signal calculator that
calculates a color reference signal representing the display brightness
of the white sub pixel for use in controlling the red sub pixel, the
green sub pixel and the blue sub pixel included in the present pixel
using the reference conversion signal corresponding to the present pixel
and the adjustment factor stored in the factor storage; a white
correction signal generator that generates a white correction signal for
use in controlling the red sub pixel, the green sub pixel and the blue
sub pixel included in the present pixel using the color reference signal
calculated by the color reference signal calculator, the upper limit
value calculated by the upper limit value calculator, and the lower limit
value calculated by the lower limit value calculator; and a color control
signal generator that generates a red output control signal, a green
output control signal and a blue output control signal for respectively
controlling display brightnesses of the red sub pixel, the green sub
pixel and the blue sub pixel included in the present pixel using the
white correction signal generated by the white correction signal
generator, the red conversion signal, the green conversion signal and the
blue conversion signal generated by the converter, and the brightness
ratio stored in the ratio storage, and outputs the generated red output
control signal, green output control signal and blue output control
signal to the display portion.

[0101] According to the foregoing configuration, the color reference
signal calculator calculates a color reference signal representing the
display brightness of the white sub pixel for use in controlling the red
sub pixel, the green sub pixel and the blue sub pixel included in the
present pixel using the reference conversion signal corresponding to the
present pixel and the adjustment factor stored in the factor storage. The
white correction signal generator generates a white correction signal for
use in controlling the red sub pixel, the green sub pixel and the blue
sub pixel included in the present pixel using the color reference signal
calculated by the color reference signal calculator, the upper limit
value calculated by the upper limit value calculator, and the lower limit
value calculated by the lower limit value calculator. The color control
signal generator generates a red output control signal, a green output
control signal and a blue output control signal for respectively
controlling display brightnesses of the red sub pixel, the green sub
pixel and the blue sub pixel included in the present pixel using the
white correction signal generated by the white correction signal
generator, the red conversion signal, the green conversion signal and the
blue conversion signal generated by the converter, and the brightness
ratio stored in the ratio storage, and outputs the generated red output
control signal, green output control signal and blue output control
signal to the display portion. Here, when the red output control signal,
the green output control signal and the blue output control signal of the
present pixel are generated in consideration of the peripheral pixel as
with the white output control signal, there are cases where discontinuous
unevenness occurs in the video picture displayed on the display portion.
However, according to the foregoing configuration, since the red output
control signal, the green output control signal and the blue output
control signal of the present pixels are generated without consideration
to the peripheral pixel unlike the white output control signal, it is
possible to display, on the display portion, a high quality video picture
that is free of discontinuous unevenness.

[0102] Moreover, in the foregoing display device, preferably, the white
reference signal calculator calculates an interpolation conversion signal
based on interpolation calculation from the reference conversion signal
corresponding to the present pixel and the reference conversion signal
corresponding to the at least one peripheral pixel, and uses a product of
the interpolation conversion signal and the adjustment factor as the
white reference signal.

[0103] According to the foregoing configuration, the white reference
signal calculator calculates an interpolation conversion signal based on
interpolation calculation from the reference conversion signal
corresponding to the present pixel and the reference conversion signal
corresponding to the at least one peripheral pixel, and uses a product of
the interpolation conversion signal and the adjustment factor as the
white reference signal. Accordingly, the white reference signal of the
present pixel that gives consideration to the peripheral pixel can be
easily obtained based on interpolation calculation.

[0104] Moreover, in the foregoing display device, preferably, the
converter generates, when the brightness ratio stored in the ratio
storage is k, the red conversion signal, the green conversion signal and
the blue conversion signal by respectively multiplying a red input
signal, a green input signal and a blue input signal as the input video
signal by (1+k), and the upper limit value calculator sets, when a
maximum value of brightness that can be displayed by each of the sub
pixels is defined as a brightness maximum value, a conversion signal with
a minimum signal level among the red conversion signal, the green
conversion signal and the blue conversion signal as the upper limit value
in a case where none of the red conversion signal, the green conversion
signal and the blue conversion signal exceeds the brightness maximum
value, and sets the brightness maximum value as the upper limit value in
a case where at least one of the red conversion signal, the green
conversion signal and the blue conversion signal exceeds the brightness
maximum value.

[0105] According to the foregoing configuration, the converter generates,
when the brightness ratio stored in the ratio storage is k, the red
conversion signal, the green conversion signal and the blue conversion
signal by respectively multiplying a red input signal, a green input
signal and a blue input signal as the input video signal by (1+k). The
upper limit value calculator sets, when a maximum value of brightness
that can be displayed by each of the sub pixels is defined as a
brightness maximum value, a conversion signal with a minimum signal level
among the red conversion signal, the green conversion signal and the blue
conversion signal as the upper limit value in a case where none of the
red conversion signal, the green conversion signal and the blue
conversion signal exceeds the brightness maximum value, and sets the
brightness maximum value as the upper limit value in a case where at
least one of the red conversion signal, the green conversion signal and
the blue conversion signal exceeds the brightness maximum value.
Accordingly, the upper limit value can be suitably obtained since the
minimum conversion signal among the red conversion signal, the green
conversion signal and the blue conversion signal or the brightness
maximum value is used as the upper limit value of the display brightness
of the white sub pixel.

[0106] Moreover, in the foregoing display device, preferably, the lower
limit value calculator sets, when a maximum value of brightness that can
be displayed by each of the sub pixels is defined as a brightness maximum
value, zero as the lower limit value in a case where none of the red
conversion signal, the green conversion signal and the blue conversion
signal exceeds the brightness maximum value, and sets a surplus relative
to the brightness maximum value of a conversion signal which exceeds the
brightness maximum value most as the lower limit value in a case where at
least one of the red conversion signal, the green conversion signal and
the blue conversion signal exceeds the brightness maximum value.

[0107] According to the foregoing configuration, the lower limit value
calculator sets, when a maximum value of brightness that can be displayed
by each of the sub pixels is defined as a brightness maximum value, zero
as the lower limit value in a case where none of the red conversion
signal, the green conversion signal and the blue conversion signal
exceeds the brightness maximum value. Accordingly, it is possible to
prevent the level of the lower limit value from becoming too high.
Moreover, the lower limit value calculator sets a surplus relative to the
brightness maximum value of a conversion signal which exceeds the
brightness maximum value most as the lower limit value in a case where at
least one of the red conversion signal, the green conversion signal and
the blue conversion signal exceeds the brightness maximum value.
Accordingly, the surplus in which the conversion signal exceeds the
brightness maximum value, that is, the brightness that cannot be
represented with the sub pixels can be compensated with the display
brightness of the white sub pixel, and the increase in the brightness of
the display portion by the white sub pixel can be realized favorably.

[0108] A display method according to the present invention is a display
method for use in a display device having a display portion that includes
pixels each having a red sub pixel, a green sub pixel, a blue sub pixel
and a white sub pixel for respectively displaying a red color, a green
color, a blue color and a white color, and displays a video picture
corresponding to an input video signal, the method includes: a conversion
step of converting the video signal using a brightness ratio, which is a
quotient obtained by dividing a brightness that can be displayed with the
white sub pixel by a brightness that can be displayed with three-color
sub pixels of the red sub pixel, the green sub pixel and the blue sub
pixel, to generate a red conversion signal, a green conversion signal and
a blue conversion signal; an upper limit value calculation step of
calculating an upper limit value of a display brightness of the white sub
pixel using the red conversion signal, the green conversion signal and
the blue conversion signal generated in the conversion step and the
brightness ratio; a lower limit value calculation step of calculating a
lower limit value of the display brightness of the white sub pixel using
the red conversion signal, the green conversion signal and the blue
conversion signal generated in the conversion step and the brightness
ratio; and a white control signal generation step of generating a white
output control signal for controlling the display brightness of the white
sub pixel such that the display brightness is not more than the upper
limit value calculated in the upper limit value calculation step and not
less than the lower limit value calculated in the lower limit value
calculation step, and outputting the generated white output control
signal to the display portion.

[0109] According to the foregoing configuration, the conversion step
converts the video signal, using a brightness ratio, to generate a red
conversion signal, a green conversion signal and a blue conversion
signal. The upper limit value calculation step calculates an upper limit
value of a display brightness of the white sub pixel using the red
conversion signal, the green conversion signal and the blue conversion
signal and the brightness ratio. The lower limit value calculation step
calculates a lower limit value of the display brightness of the white sub
pixel using the red conversion signal, the green conversion signal and
the blue conversion signal and the brightness ratio. The white control
signal generation step generates a white output control signal for
controlling the display brightness of the white sub pixel such that the
display brightness is not more than the upper limit value and not less
than the lower limit value, and outputs the generated white output
control signal to the display portion. Accordingly, the display
brightness of the white sub pixel can be controlled to be an appropriate
level that is not more than the upper limit value and not less than the
lower limit value. As a result, it is possible to display a video picture
in which black vertical streaks are unnoticeable.

[0110] According to the present invention, since a white output control
signal for controlling the display brightness of the white sub pixel such
that the display brightness is not more than the upper limit value and
not less than the lower limit value is generated and output to the
display portion, the display brightness of the white sub pixel can be
controlled to be an appropriate level. Accordingly, it is possible to
display a video picture in which black vertical streaks are unnoticeable.

INDUSTRIAL APPLICABILITY

[0111] The display device and the display method according to the present
invention are effective as a device and a method for suitably displaying
color video pictures on various types of displays such as a liquid
crystal display, a plasma display panel, and an organic EL display.